Heat-resistant magnesium alloy

ABSTRACT

A heat-resistant magnesium alloy according to the present invention includes Mg, a major component; a first alloying element “M 1 ” being any one or more members that are selected from the group consisting of Al and Ni; a second alloying element “M 2 ” being any one or more members that are selected from the group consisting of Mn, Ba, Cr and Fe; and Ca; and it has a metallic structure including: Mg crystalline grains; plate-shaped precipitated substances being precipitated within grains of the Mg crystalline grains; and grain-boundary crystallized substances being crystallized at grain boundaries between the Mg crystalline grains to form networks that are continuous microscopically. 
     Since the plate-shaped precipitated substances exist within the Mg crystalline grains, the movements of dislocation within the Mg crystalline grains are prevented, and accordingly it becomes less likely to deform. Moreover, since the grain-boundary crystallized substances, which form the networks, are present continuously microscopically at the grain boundaries between the Mg crystalline grains, the strength at the grain boundaries improves. The heat-resistant magnesium alloy according to the present invention in which both of the Mg crystalline grains&#39; granular interior and the grain boundaries between the Mg crystalline grains are strengthened exhibits high mechanical characteristics even in high-temperature regions.

TECHNICAL FIELD

The present invention is one which relates to a heat-resistant magnesiumalloy that are capable of withstanding services under high loads and athigh temperatures.

BACKGROUND ART

Magnesiumalloy, which is much more lightweight than aluminum alloy is,is about to come to be used widely for aircraft material, vehiclematerial, and the like, from the viewpoint of weight saving. However, inmagnesium alloy, since the strength and heat resistance are notsufficient depending on applications, further improvement of thecharacteristics has been sought.

Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No.2004-162,090, and in Japanese Unexamined Patent Publication (KOKAI)Gazette No. 2004-232,060, there are disclosed magnesium alloys in whichcalcium (Ca) and aluminum (Al) are contained in adequate amounts. Inthese literatures, since Ca—Al compounds and Mg—Ca compounds crystallizeor precipitate at the grain boundaries between the Mg crystalline grainsin the magnesium alloys, the movements of dislocations are held back. Asa result, the magnesium alloys undergo creep deformations less even inhigh-temperature regions, and therefore exhibit good heat resistance.Further, in the aforementioned magnesium alloys, Mn is solidified intothe Mg crystalline grains, and thereby the magnesium alloys aresubjected to solid-solution strengthening.

DISCLOSURE OF THE INVENTION

The metallic structure of alloy affects its characteristics greatly.Accordingly, in order to obtain a magnesium alloy that possessesstrength and creep resistance being sufficient for services at hightemperatures, it is necessary to adapt the types and amounts of additiveelements into adequate ones in order to control the metallic structure.

It is an object of the present invention to provide a magnesium alloy,both of whose crystalline grains' interior and crystalline grainboundaries are strengthened and which therefore exhibits good heatresistance, by means of controlling the metallic structure of themagnesium alloy using adequate alloying elements.

Specifically, a heat-resistant magnesium alloy according to the presentinvention is characterized in that it includes:

magnesium (Mg), a major component;

a first alloying element “M1” being any one or more members that areselected from the group consisting of aluminum (Al) and nickel (Ni);

a second alloying element “M2” being any one or more members that areselected from the group consisting of manganese (Mn), barium (Ba),chromium (Cr) and iron (Fe); and

calcium (Ca); and

it has a metallic structure including:

Mg crystalline grains;

plate-shaped precipitated substances being precipitated within grains ofthe Mg crystalline grains; and

grain-boundary crystallized substances being crystallized at grainboundaries between the Mg crystalline grains to form networks that arecontinuous microscopically.

Note that, in the present description, the “networks that are continuousmicroscopically” take on network structures (three-dimensionally meshstructures) macroscopically, and are states in which crystals existcontinuously even inside the networks (see FIG. 2). Therefore, thefollowing are not involved: discontinuous states whose interior isconstituted of small crystals, even though they take on networkstructures (see FIG. 3).

Since the heat-resistant magnesium alloy according to the presentinvention includes the second alloying element “M2,” it has theplate-shaped precipitated substances within the grains of the Mgcrystalline grains, and the grain-boundary crystallized substances,which form the networks that are continuous microscopically, at thegrain boundaries, as will be detailed later. Since the plate-shapedprecipitated substances exist within the Mg crystalline grains, themovements of dislocation within the Mg crystalline grains are prevented,and accordingly it becomes less likely to deform. Moreover, since thegrain-boundary crystallized substances, which form the networks, arepresent continuously microscopically at the grain boundaries between theMg crystalline grains, the strength at the grain boundaries improves. Asa result, the heat-resistant magnesium alloy according to the presentinvention exhibits high mechanical characteristics even inhigh-temperature regions. That is, in the magnesium alloy according tothe present invention, the mechanical characteristics inhigh-temperature regions are improved by strengthening it not onlywithin the Mg crystalline grains' granular interior but also at thegrain boundaries between the Mg crystalline grains.

Said precipitated substances can desirably comprise a Laves-phasecompound with type-“C15” crystalline structure. Moreover, saidprecipitated substances can desirably be precipitated parallel to the{001} plane of Mg crystal.

Said grain-boundary crystallized substances, which form the networksthat are continuous microscopically, can desirably comprise anMg-“M1”-Ca-system compound. Moreover, said grain-boundary crystallizedsubstances can desirably comprise a mixed-crystal phase of a Laves-phasecompound with type-“C14” crystalline structure and a Laves-phasecompound with type-“C36” crystalline structure; on this occasion, it isallowable that said mixed-crystal structure can include the type-“C14”crystalline structure more than the type-“C36” crystalline structure.

When the precipitated substances are precipitated parallel to the {001}plane of Mg crystal, the movements of dislocation on the sliding planeof hexagonal Mg crystal are suppressed. When the grain-boundarycrystallized substances comprise a mixed-crystal phase of a Laves-phasecompound with type-“C14” crystalline structure and a Laves-phasecompound with type-“C36” crystalline structure, compounds, whichconstitute the networks, do not undergo any phase separation, andconsequently turn into single crystals virtually in appearance (see FIG.4), the area of the crystalline-grain boundaries between crystallinegrains that constitute the networks, and the number of the crystallinegrains that constitute the networks become minimum.

Note that the aforementioned “type-‘C14’,” “type-‘C15’,” and“type-‘C36’” are codes in accordance with a magazine,“STRUKTURBERICHTE,” and express three similar basic crystallinestructures that are represented by MgZn₂, MgCu₂ and MgNi₂ of the Lavesphases.

Further, it is desirable that it can have fine particles that includesaid second alloying element “M2” within said Mg crystalline grains.

The heat-resistant magnesium alloy according to the present inventioncan preferably include: Ca in an amount of from 2% by mass or more to 4%by mass or less; said first alloying element “M1” in an amount of from0.9 or more to 1.1 or less by mass ratio with respect to Ca (“M1”/Ca);said second alloying element “M2” in an amount of from 0.3% by mass ormore to 0.6% by mass or less; and the balance comprising Mg andinevitable impurities; when the entirety is taken as 100% by mass.

Alternatively, the heat-resistant magnesium alloy according to thepresent invention can preferably include: Ca in an amount of from 1.235atomic % or more to 2.470 atomic % or less; said first alloying element“M1” in an amount of from 1.34 or more to 1.63 or less by atomic ratiowith respect to Ca (“M1”/Ca); said second alloying element “M2” in anamount of from 0.13 atomic % or more to 0.27 atomic % or less; and thebalance comprising Mg and inevitable impurities; when the entirety istaken as 100 atomic %.

Heat-resistance magnesium alloys, which possess metallic structures thatare desirable from the viewpoints of mechanical characteristics at hightemperatures, are obtainable by setting the content proportions of thefirst alloying element, second alloying element and Ca that theheat-resistance magnesium alloy according to the present inventioncontains to appropriate ranges.

Note that the “heat resistance” being referred to in the presentspecification is one that is evaluated by mechanical properties ofmagnesium alloy in high-temperature atmospheres (creep characteristicsor high-temperature strengths that are determined by means of stressrelaxation tests or axial-force retention tests, for instance).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a metallic-structure photograph in which a cross section of atest specimen being labeled #01 was observed with a metallographicmicroscope.

FIG. 2 is a metallic-structure photograph in which an observationalsample being labeled #01 was observed with a transmission electronmicroscope (or TEM).

FIG. 3 is a metallic-structure photograph in which an observationalsample being labeled #C1 was observed with a TEM.

FIG. 4 is a dark-field scanning-transmission-electron-microscope (orDF-STEM) image on the observational sample being labeled #01.

FIG. 5 is a DF-STEM image on the observational sample being labeled #C1.

FIG. 6 is a TEM image on the observational sample being labeled #01, andan electron diffraction pattern thereof (the incident direction being<110>).

FIG. 7 is another TEM image on the observational sample being labeled#01, and another electron diffraction pattern thereof (the incidentdirection being <111>).

FIG. 8 is a TEM image on the observational sample being labeled #C1, andan electron diffraction pattern thereof (the incident direction being<111>).

FIG. 9 is a DF-STEM image in which the interior of Mg crystalline grainsin the observational sample being labeled #01 was observed.

Note that “#01” and “#C1” are codes for distinguishing magnesium alloyswhose compositions differed in later-described examples.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out the heat-resistant magnesiumalloy according to the present invention (hereinafter being abbreviatedto as “magnesium alloy”) will be explained.

The magnesium alloy according to the present invention includes:magnesium (Mg), a major component; a first alloying element “M1”; asecond alloying element “M2”; and calcium (Ca); and it has a metallicstructure that includes: Mg crystalline grains; plate-shapedprecipitated substances being precipitated within grains of Mgcrystalline grains; and grain-boundary crystallized substances beingcrystallized at grain boundaries between the Mg crystalline grains toform networks that are continuous microscopically.

In the magnesium-alloy according to the present invention, theplate-shaped precipitated substances are present within the Mgcrystalline grains. The plate-shaped precipitated substances prevent themovements of dislocation within the Mg crystalline grains. Thedeformations of crystal occur because the dislocation moves on slidingplane. Therefore, it is allowable that they can be plate-shapedprecipitated substances that are parallel to the “c” plane of hexagonalMg crystal, that is, to the {001} plane of Mg crystal. Note that theplate-shaped precipitated substances come to exhibit a plate thicknessof 2-20 nm, and that the thicker the plate thickness is the moremechanical characteristics improve.

Moreover, it is allowable that the plate-shaped precipitated substancescan comprise a Laves-phase compound with type-“C15” crystallinestructure. The “c” plane of Mg crystal, and the {111} plane of “C15”structure are likely to form interfaces that are stable to each othercrystallographically, and therefore it is possible to predict that theformation of the plate-shaped precipitated substances is facilitated. Itis allowable that compounds constituting the precipitated substancesthat have such crystalline structures can be “M1”-Ca-system compoundsand/or Mg-“M1”-Ca-system compounds.

It is allowable as well that the magnesium alloy according to thepresent invention can have fine particles within the granular interiorof the Mg crystalline grains. The fine particles are present within theMg crystalline grains, and most of them exist around the plate-shapedprecipitated substances. It is believed that, although these fineparticles are present within the Mg crystalline grains, they are notthose which contribute to the improvement of strength inside the Mgcrystalline grains. However, the presence of the fine particles isrelated to the generation of the precipitated substances (will bedescribed later), and the fine particles are fine particles, whichinclude “M2,” like “M1”-“M2”-system compounds, for instance. Note thatthe fine particles are sphere-shaped ones substantially and exhibitparticle diameters of 10-15 nm approximately.

In the magnesium alloy according to the present invention, thegrain-boundary crystallized substances, which form networks that arecontinuous microscopically, are crystallized at the grain boundariesbetween the Mg crystalline grains to be present therein. For example,even in compositions being made by excluding the second alloying element“M2” from that of the magnesium alloy according the present invention,grain-boundary crystallized substances might be crystallized at thegrain boundaries between the Mg crystalline grains, and additionallymight form networks. However, in magnesium alloys that do not includeany “M2,” it has been understood that no microscopic continuity can beseen in the grain-boundary crystallized substances that form thenetworks. On the other hand, in the magnesium alloy according to thepresent invention, because of including “M2,” the grain-boundarycrystallized substances form the networks that are continuousmicroscopically. Because of the fact that the networks are continuousmicroscopically, the crystalline grain-boundary area of compounds thatconstitute the networks, and the number of crystalline grains arereduced greatly. As a result, the grain-boundary strength is improved,and is then strengthened. On this occasion, it is desirable that thenetworks of the grain-boundary crystallized substances can cover 70% ormore of the grain boundaries between the Mg crystalline grains that areobserved linearly in a regional cross section with 400 μm×600 μmapproximately in the magnesium alloy (this value will be abbreviated toas a “covering ratio of networks”).

Moreover, it is allowable that the grain-boundary crystallizedsubstances can comprise a mixed-crystal phase of a Laves-phase compoundwith type-“C14” crystalline structure and a Laves-phase compound withtype-“C36” crystalline structure. The type-“C14” crystalline structure,and the type-“C36” crystalline structure are desirable, because they arehexagonal ones to each other and are likely to form mixed-phases. Sincethe Laves-phase compounds in the mixed-crystal phase come to beapproximated to the single crystals extremely, the grain-boundarycrystallized substances are continuous microscopically; and accordinglythe crystalline-grain boundary area of crystalline grains, that is,compounds that constitute the networks, and the number of thecrystalline grains that constitute the networks become minimum.

Moreover, it is desirable that the grain-boundary crystallizedsubstances can comprise an Mg-“M1”-Ca-system compound. Since Mg₂Ca has atype-“C14” crystalline structure, it is assumed that a mixed-crystalphase of a type-“C14” crystalline structure and a type-“C36” crystallinestructure is formed by solidifying “M1” into Mg₂Ca. In this instance, itis allowable that the mixed-crystal phase can include the type-“C14”crystalline structure more than the type-“C36” crystalline structure.

The magnesium alloy according to the present invention that has themetallic structure as described above includes: magnesium (Mg), a majorcomponent; a first alloying element “M1”; a second alloying element“M2”; and calcium (Ca).

For the first alloying element “M1,” it is possible to use at least onemember that is selected from the group consisting of aluminum (Al) andnickel (Ni). Although not only Al but also Ni are elements that reactwith Ca to form compounds and take on a type-“C15” Laves structure, amixed-crystal phase of a type-“C14” Laves structure and a type-“C36”Laves structure is formed under such a condition that Mg₂Ca, which takeson a type-“C14” Laves structure, is dominant, because Al and/or Ni aredissolved into Mg₂Ca For the second alloying element “M2,” it ispossible to use at least one member that is selected from the groupconsisting of manganese (Mn), barium (Ba), chromium (Cr) andiron (Fe).The reason why it is possible to use these elements as “M2” can beexplained by means of structural changes of the magnesium alloyaccording to the present invention in the cooling process.

It was understood from the cooling curve when casting a cast productcomprising the magnesium alloy according to the present invention by ageneral solidifying process (air cooling) that three temperature-haltingpoints (the respective temperatures are labeled “T1,” “T2” and “T3”; and“T1”>“T3,” and “T2”>“T3”) appear. When the molten-metal temperaturereaches a primary-crystal temperature (i.e., a temperature at which thesolidification begins: “T1”=from 600° C. or more to 620° C. or less),primary-crystal Mg crystallized. Moreover, when it reaches “T2,” it ispredicted that “M1” and “M2” react to generate fine particles of“M1”-“M2”-system compounds, high-temperature-generated compounds. Next,when it reaches the eutectic temperature “T3,” the grain-boundarycrystallized substances, which form the networks, crystallize along witheutectic Mg. However, as a result of Carrying out an elementary analysison the fine particles of the resulting cast product, it was found that“M2” was included therein more than the theoretical value. Specifically,in regions of low temperatures that are much lower than “T3,” it ispossible to predict that “M1” is spewed out from the fine particles (or“M1”-“M2”-system compounds), and that the spewed-out “M1” formscompounds with Ca and then precipitates being accompanied by theagglomeration of Ca that dissolves into the Mg crystalline grains.

Therefore, it is necessary that not only the second alloying element“M2” can react with the first alloying element “M1” at high temperaturesthat are higher than “T3” but also it can be less likely to dissolveinto Mg. Because of such reasoning, it is possible to use at least onemember that is selected from the group consisting of manganese (Mn),barium (Ba), chromium (Cr) and iron (Fe), especially from among thetransition elements. These elements exhibit atomic radii beingcomparable with each other, and take on similar crystalline structures;further they react with “M1” to generate the compounds in comparativelyhigh-temperature regions, to be concrete, between “T1” and “T3” alone.

Note that the magnesium alloy according to the present inventionincludes at least one species of the aforementioned first alloyingelements and second alloying elements, respectively. It is alsoallowable that it can include one species of them as for the firstelement and second element, respectively; and it is even allowable thatit can include plural species of them as for either one of them or bothof them.

It is preferable that the magnesium alloy according to the presentinvention can include: Ca in an amount of from 2% by mass or more to 4%by mass or less; said first alloying element “M1” in an amount of from0.9 or more to 1.1 or less by mass ratio with respect to Ca (“M1”/Ca);said second alloying element “M2” in an amount of from 0.3% by mass ormore to 0.6% by mass or less; and the balance comprising Mg andinevitable impurities; when the entirety is taken as 100% by mass.Alternatively, it is preferable that the magnesium alloy according tothe present invention can include: Ca in an amount of from 1.235 atomic% or more to 2.470 atomic % or less; said first alloying element “M1” inan amount of from 1.34 or more to 1.63 or less by atomic ratio withrespect to Ca (“M1”/Ca); said second alloying element “M2” in an amountof from 0.13 atomic % or more to 0.27 atomic % or less; and the balancecomprising Mg and inevitable impurities; when the entirety is taken as100 atomic %

When “M1”/Ca is less than 0.9 by mass ratio (namely, being less than1.34 by atomic ratio), it is not preferable because the content of Ca isso great that the castability deteriorates. On the other hand, when“M1”/Ca surpasses 1.1 by mass ratio (namely, surpassing 1.63 by atomicratio), it is not preferable because the grain-boundary crystallizedsubstances are less likely to turn into a mixed-crystal phase, andbecause crystalline grains, which are constituted of type-“C36” Lavesstructure alone, are likely to be formed so that they undergo phaseseparation. Further, when type-“C36” crystalline structure is exposed tohigh temperatures, it is likely to undergo phase transition totype-“C15” crystalline structure (Scripta Materialia 51 (2004)1005-1010). Since the type-“C15” crystalline structure is likely toundergo massive agglomeration in high-temperature regions, and since itdoes not form the networks of the crystallized substances, networkswhich are continuous microscopically, the mechanical characteristics athigh temperatures lower remarkably. A more preferable “M1”/Ca value canbe from 0.95 or more to 1.05 or less (namely, being 1.42-1.56 by atomicratio).

When the content proportion of the second alloying element “M2” is lessthan 0.3% by mass (namely, 0.13 atomic %), it is not preferable becauseit is impossible to retain the “M1,” which constitutes the precipitatedsubstances in the cooling step (or solidifying step), as compounds sothat the precipitated substances are not precipitated sufficiently.Moreover, it is not preferable because many “M1” reside without evercombining with “M2” so that crystalline grains, which possess type-“C36”Laves structure alone that does not take on any mixed-crystal structureas the grain-boundary crystallized substances, are likely to be formed,and so that they undergo phase separation. On the other hand, when thecontent proportion of “M2” surpasses 0.6% by mass (namely, 0.27 atomic%), it is not preferable because compounds that contain “M2” areprecipitated within the grain-boundary crystallized substances so thatthey might possibly cut off the networks. The lower limit of a morepreferable content proportion of “M2” can be 0.34% by mass (namely, 0.15atomic %) or more. The upper limit of a more preferable contentproportion of “M2” can be 0.55% by mass (namely, 0.25 atomic %) or less,and can much more preferably be 0.5% by mass (namely, 0.23 atomic %) orless.

Ca is an element that forms type-“C14” and type-“C36” Laves structurestogether with Mg. When a content proportion of Ca is less than 2% bymass (namely, 1.235 atomic %), it is not preferable because theprecipitated substances and grain-boundary crystallized substances arenot generated sufficiently so that the effect of improving theheat-resistant characteristic is not sufficient. On the other hand, whenthe content proportion of Ca surpasses 4% by mass (namely, 2.470 atomic%), it is not preferable because the generation amounts of theprecipitated substances and grain-boundary crystallized substancesbecome too great so that problems might arise in post-processes. A morepreferable content proportion of Ca can be from 2.5% by mass or more to3.5% by mass or less (namely, from 1.54 atomic % or more to 2.16 atomic% or less).

The magnesium alloy according to the present invention is not limited tothose made by ordinary gravity casting and pressure casting, but caneven be those made by die-cast casting. Moreover, even the casting moldbeing utilized for the casting does not matter if it is sand molds,metallic molds, and the like. Although even the solidification rate inthe solidifying step is not limited in particular, it is allowable tolet it stand to cool in air atmosphere.

Beginning with the fields of space, military and aviation, applicationsof the magnesium alloy according to the present invention can beextended to various fields, such as automobiles and home electricinstruments. In reality, however, it is all the more suitable that,taking advantage of its heat resistance, the magnesium alloy accordingto the present invention can be utilized in products being utilized inhigh-temperature environments, such as engines, transmissions,compressors for air conditioner or their related products that are putin place within the engine room of automobile, for instance. To beconcrete, the following can be given: cylinder heads, cylinder blocksand oil pans of internal combustion engine; impellers for turbochargerof internal combustion engine, transmission cases being used forautomobile and the like, and so forth.

So far, the embodiment modes of the heat-resistant magnesium alloyaccording to the present invention have been explained, however, thepresent invention is not one which is limited to the aforementionedembodiment modes. It can be conducted in various modes to whichmodifications, improvements, and the like, which one of ordinary skillin the art can carry out, are performed, within a range not departingfrom the scope of the present invention.

Hereinafter, while giving specific examples, the present invention willbe explained in detail.

Two kinds of test specimens whose contents (or addition amounts) of Al,Ca and Mn in magnesium alloys were varied were made, and then not onlytheir metallic structures were observed but also a stress relaxationtest was carried out.

(Making of Test Specimens)

A chloride-system flux was coated onto the inner surface of a cruciblebeing made of iron that had been preheated within an electric furnace,and then a weighed pure magnesium base metal, pure Al, and an Mg—Mnalloy, if needed, were charged into it and were then melted. Further,weighed Ca was added into this molten metal that was held at 750° C.(i.e., a molten-metal preparing step).

After fully stirring this molten metal to melt the raw materialscompletely, it was held calmly at the same temperature for a while.During this melting operation, a mixture gas of carbon dioxide gas andSF₆ gas was blown onto the molten metal's surface in order to preventthe burning of Mg, and the flux was sprayed whenever being deemedappropriate.

The thus obtained various alloy molten metals were poured into ametallic mold with a predetermined configuration (i.e., a molten-metalpouring step), and were then solidified in air atmosphere (i.e., asolidifying step). Thus, test specimens with 30 mm×300 mm×40 mm weremade by means of gravity casting. The obtained test specimens werelabeled #01 (an example including Mn), and #C1 (a comparative examplenot including Mn). The chemical compositions of the respective testspecimens were specified in Table 1. Note that, in the magnesium-alloycompositions being given in Table 1, the balances are Mg, respectively.

TABLE 1 Magnesium-alloy Magnesium-alloy Composition Composition (% bymass) (atomic %) Test Specimen Al Ca Mn Al Ca Mn Al/Ca #01 3 3 0.5 2.751.85 0.23 1.49 #C1 3 3 — 2.74 1.85 — 1.48

Note that, in Table 1, “% by mass” and “atomic %” are used as the unitsfor the alloy compositions being labeled #01 and #C1. Here, the valuesthat used the unit, “% by mass,” were the charged quantities in themolten-metal preparing step, and those values were converted into the“atomic %.”

(Observation on Metallic Structure)

Test specimens #01 and #C1 were observed with a metallographicmicroscope or transmission electron microscope (TEM).

FIG. 1 is a metallic-structure photograph in which a cross section ofthe test specimen being labeled #01 was observed with a metallographicmicroscope. The Mg crystalline grains (bright parts), and thegrain-boundary crystallized substances (black parts) that existed likenetworks at the grain boundaries between the Mg crystalline grains wereobserved. Note that, although not being shown diagrammatically, ametallic-structure photograph being similar to FIG. 1 was obtained evenwhen a cross section of the test specimen being labeled #C1 wasobserved. That is, in either one of the test specimens, network-shapedgrain-boundary crystallized substances were observed macroscopically.

Next, in order to observe micro-fine constructions of the metallicstructures, the respective test specimens were adapted into aflake-shaped observational sample, respectively, and were then observedusing a TEM.

FIG. 2 and FIG. 3 are metallic-structure photographs in which theobservational samples being labeled #01 and #C1 were observed with theTEM. In both of them, crystalline grain boundaries in which two or morecrystalline grains of primary-crystal Mg neighbor to each other wereobserved. In FIG. 2 (#01), the grain-boundary crystallized substances(black parts) were grown as a lamellar shape, and were continuous. InFIG. 3 (#C1), the grain-boundary crystallized substances wereinterrupted partially, and were discontinuous. Note that the coveringratio of networks in #01 was about 90%.

Moreover, FIG. 4 and FIG. 5 are a dark-fieldscanning-transmission-electron-microscope (DF-STEM) images in which thegrain-boundary crystallized substances in the observational samplesaccording to #01 and #C1 were observed, respectively. In the testspecimen being labeled #01, no phase separation was seen as shown inFIG. 4; whereas, in the test specimen being labeled #C1, phaseseparation was seen as shown in FIG. 5. When an elementary mapping wascarried out with respect to the DF-STEM images of FIG. 4 and FIG. 5 bymeans of energy-dispersion-type X-ray spectroscopy (EDX), Mg, Al and Cawere distributed uniformly in FIG. 4 (#01); whereas the concentration ofAl was high in the crystalline grains, which were agglomeratedgranularly to undergo phase separation, in FIG. 5 (#C1). And, theelectron diffraction of type-“C36” crystalline structure was obtainedfrom the crystalline grains with high Al concentrations. On the otherhand, the electron-diffraction pattern of type-“C14” crystallinestructure was obtained mainly from the crystals in which each of Mg, Aland Ca was distributed uniformly in FIG. 4 and FIG. 5; however, thediffraction spot of type-“C36” crystalline structure, which coincidedwith the twofold cycle to type-“C14” crystalline structure, appearedpartially, even though they did not undergo any phase separation.Specifically, it was understood that the crystals in which Mg, Al and Cawere distributed uniformly were a mixed-crystal phase of type-“C14”crystalline structure and type-“C36” crystalline structure, and werevirtually single crystals visually. Therefore, in the test specimensbeing labeled #01, the grain-boundary crystallized substances formingthe networks were continuous microscopically, and they virtually turnedinto single crystals visually. On the contrary, in the test specimenbeing labeled #C1, although the grain-boundary crystallized substancesformed networks macroscopically, the networks were discontinuousmicroscopically, and the Laves-phase compounds, which comprisedtype-“C36” crystalline structure alone and had undergone phaseseparation, were present.

Note that, on a magnesium alloy in which the Mn content in #01 waschanged to 0.2% by mass (namely, 0.09 atomic %), the grain-boundarycrystallized substances were observed with the TEM, though not beingshown diagrammatically. According to the obtained DF-STEM image, themassive agglomerations that were seen in #C1 (FIG. 5) decreased so thatcompounds extending as strip shapes came to account for it greatly whenthe Mn amount increased; however, it was understood that no continuitythat was observed in #01 (FIG. 4) was seen when the Mn content was 0.2%by mass.

FIG. 6 and FIG. 7 are TEM images on Test Specimen #01, and FIG. 8 is aTEM image on Test Specimen #C1. In FIG. 6, the interior of the Mgcrystalline grains was observed while setting the incident direction to<110>, whereas it was observed while setting the incident angle to <111>in FIG. 7 and FIG. 8. In FIG. 6 (#01), streak-shaped precipitatedsubstances that were parallel to the {001} plane were seen. And, fromFIG. 7 in which the observation was carried out at the same position asthat in FIG. 6 but while inclining the incident direction, theprecipitated substances were found to have plate shapes that wereparallel to the {001} plane. When the STEM-EDX analysis was carried outonto these plate-shaped precipitated substances, Al and Ca were detectedmainly. Moreover, from the plate-shaped precipitated substances, theelectron-diffraction pattern of type-“C15” crystalline structure thatcoincided with Al₂Ca was obtained.

On the contrary, in FIG. 8 (#C1), no clear streak-shaped contrast wasseen. Note that, even when the same STEM-EDX analysis as that was donefor #01 was carried out, Al and Ca were hardly detected. Therefore, theprecipitated substances hardly existed in the test specimen beinglabeled #C1.

FIG. 9 is a DF-STEM image in which the interior of the Mg crystallinegrains in the observational sample being labeled #01 was observed. Aplurality of fine particles were seen around the plate-shapedprecipitated substances. When an elementary analysis was carried outonto the fine particles (e.g., “B” in FIG. 9), Mn was detected. Notethat no Mn was detected even when the plate-shaped precipitatedsubstances (e.g., “A” in FIG. 9) were analyzed.

(Stress Relaxation Test)

A stress relaxation test was carried out not only onto Test Specimens#01 and #C1 given in Table 1 but also onto AXE662, AE42 and AZ91D (allas per ASTM standards), thereby examining the magnesium alloys' heatresistance (e.g., creep resistance). In the stress relaxation test, aprocess was measured, process in which the stress, which was needed whena load was applied to a test specimen until it exhibited a predetermineddeformation magnitude, decreased with time in the course of testingtime. To be concrete, in 150° C. air atmosphere, a compression stress of100 MPa was loaded to the test specimens, and then the compressionstress was lowered in agreement with the elapse of time so as to keepthe displacements of the test specimens at that time constant.

In Table 2 and Table 3, the respective test specimens' alloycompositions, and their stresses after 40 hours since the stressrelaxation test started are given. Note that, in the magnesium-alloycompositions being given in Table 2 and Table 3, the balances are Mg,respectively. Moreover, “RE” is a mish metal.

TABLE 2 Magnesium-alloy Test Composition (% by mass) Stress Specimen AlZn Ca RE Mn (MPa) #01 3 — 3 — 0.5 92 #C1 3 — 3 — — 86 AXE662 6 — 6 2 —83 AE42 4 — — 2 — 74 AZ91D 9 1 — — — 68

TABLE 3 Magnesium-alloy Test Composition (atomic %) Stress Specimen AlZn Ca RE Mn (MPa) #01 2.75 — 1.85 — 0.23 92 #C1 2.74 — 1.85 — — 86AXE662 5.67 — 3.81 0.36 — 83 AE42 3.68 — — 0.36 — 74 AZ91D 8.23 0.38 — —— 68

Test Specimen #01 exhibited a decrease proportion of the loaded stressespecially less, compared with those of the other test specimens, andtherefore showed high creep resistance even under high temperatures.This is due to the following: the firm networks, which were continuousmicroscopically, were formed at the grain boundaries between the Mgcrystalline grains because of the presence of Mn; and the deformationresistance of Test Specimen #01 enlarged so that the strength thereofimproved because the movements of dislocation were suppressed by theplate-shaped precipitated substances within the Mg crystalline grains.

1. A heat-resistant magnesium alloy being characterized in that itincludes: magnesium (Mg), a major component; a first alloying element“M1” being any one or more members that are selected from the groupconsisting of aluminum (Al) and nickel (Ni); a second alloying element“M2” being any one or more members that are selected from the groupconsisting of manganese (Mn), barium (Ba), chromium (Cr) and iron (Fe);and calcium (Ca); and it has a metallic structure including: Mgcrystalline grains; plate-shaped precipitated substances beingprecipitated within grains of the Mg crystalline grains; andgrain-boundary crystallized substances being crystallized at grainboundaries between the Mg crystalline grains to form networks that arecontinuous microscopically.
 2. The heat-resistant magnesium alloy as setforth in claim 1, wherein said precipitated substances comprise aLaves-phase compound with type-“C15” crystalline structure.
 3. Theheat-resistant magnesium alloy as set forth in claim 1, wherein saidprecipitated substances are precipitated parallel to the {001} plane ofMg crystal.
 4. The heat-resistant magnesium alloy as set forth in claim1, wherein said grain-boundary crystallized substances comprise anMg-“M1”-Ca-system compound.
 5. The heat-resistant magnesium alloy as setforth in claim 1, wherein said grain-boundary crystallized substancescomprise a mixed-crystal phase of a Laves-phase compound with type-“C14”crystalline structure and a Laves-phase compound with type-“C36”crystalline structure.
 6. A heat-resistant magnesium alloy as set forthin claim 5, wherein said mixed-crystal structure includes the type-“C14”crystalline structure more than the type-“C36” crystalline structure. 7.The heat-resistant magnesium alloy as set forth in claim 1 having fineparticles that include “M2” within said Mg crystalline grains.
 8. Theheat-resistant magnesium alloy as set forth in claim 1 including: Ca inan amount of from 2% by mass or more to 4% by mass or less; said firstalloying element “M1” in an amount of from 0.9 or more to 1.1 or less bymass ratio with respect to Ca (“M1”/Ca); said second alloying element“M2” in an amount of from 0.3% by mass or more to 0.6% by mass or less;and the balance comprising Mg and inevitable impurities; when theentirety is taken as 100% by mass.
 9. The heat-resistant magnesium alloyas set forth in claim 8 including said second alloying element “M2” inan amount of from 0.3% by mass or more to 0.5% by mass or less.
 10. Theheat-resistant magnesium alloy as set forth in claim 1 including: Ca inan amount of from 1.235 atomic % or more to 2.470 atomic % or less; saidfirst alloying element “M1” in an amount of from 1.34 or more to 1.63 orless by atomic ratio with respect to Ca (“M1”/Ca); said second alloyingelement “M2” in an amount of from 0.13 atomic % or more to 0.27 atomic %or less; and the balance comprising Mg and inevitable impurities; whenthe entirety is taken as 100 atomic %.
 11. The heat-resistant magnesiumalloy as set forth in claim 10 including said second alloying element“M2” in an amount of from 0.15 atomic % or more to 0.25 atomic % orless.
 12. The heat-resistant magnesium alloy as set forth in claim 1,wherein: said first alloying element is Al; and said second alloyingelement is Mn.